The lasers that operate in phase locked mode deliver pulses whose duration may be as short as 10 fs, even less. These pulses generally have an approximately gaussian time envelope, possibly with mainly linear or quadratic phase modulation.
In many applications, there is a need for longer and sub-structured pulses; for example, there may be a desire to generate a complex picosecond pulse consisting of a train of individual femtosecond pulses, modulated in amplitude and/or in phase and/or in polarization.
To meet this need, many laser pulse shaping techniques have been developed. The most commonly used are filtering in the Fourier plane and acousto-optic temporal modulation.
The technique of filtering in the Fourier plane is described, in particular, by the document U.S. Pat. No. 5,682,262. In such a device, a first diffraction grating (or prism) disperses the various spectral components of an input pulsed laser beam. These spectral components are focused by a cylindrical lens so as to form a two-dimensional optical field which is projected onto a programmable amplitude and phase mask. This mask makes it possible to selectively attenuate and phase-shift each component, so as to modify the spectrum of the input pulses in a predetermined manner. A second cylindrical lens and a second grating or prism recombine the filtered spectral components, to form a temporally structured pulsed laser beam at the output.
This method is extremely flexible, but does have a certain number of drawbacks:                it is complex to implement;        the method introduces space-time aberrations; this means that the temporal profile of the output pulsed beam varies spatially in a direction transversal to the direction of propagation; these space-time aberrations become particularly detrimental if the output pulsed beam is required to be strongly focused (nonlinear microscopy applications, for example);        the phase mask is pixelated, and necessarily includes opaque interstitial areas between the pixels that diffract the light; this is reflected in the appearance of temporal replicas of the output pulses;        to obtain a predetermined output temporal profile, it is necessary to accurately know the spectrum of the input pulse, as well as its spectral phase, and perform relatively complex computations; it may even be necessary to use iterative optimization techniques (genetic algorithms, in particular) whose convergence can be slow, even random.        
The article by T. Brixner and G. Gerber entitled “Femtosecond polarization pulse shaping”, Optics Letters, Vol. 26, no. 8, pages 557-559, 15 Apr. 2001, describes a variant of the technique of filtering in the Fourier plane that makes it possible to modulate the polarization of a pulse. The technique is based on the use of a spatial filter consisting of a double-layer pneumatic liquid crystal modulator.
Another shaping technique known from the prior art is based on the use of an acousto-optic modulator. Such a device is described, for example, by the article by P. Tournois: “Acousto-optic programmable dispersive filter for adaptive compensation of group delay time dispersion in laser systems”, Opt. Commun. 140, 245-249 (1997), and marketed by the company “Fastlite” under the tradename “Dazzler”. This device is based on the interaction, inside a birefringent crystal, of a laser pulse and an acoustic wave. Its main drawback, linked to the use of an acoustic wave, is its low rate of operation, a few kHz at most, whereas phase locked laser oscillators emit pulses at a rate of several MHz.
Yet another technique for shaping laser beams is direct space-to-time shaping (DST), described by the article by C. Froehly, B. Colombeau and M. Vampouille “Shaping and analysis of picosecond light pulses”, in “Progress in Optics XX”, North Holland 1983, pages 112-115. This technique is particularly simple. It consists in directing an input pulsed laser beam onto a diffraction grating, preferably “blazed” or “echelle”. The beam diffracted to the first order (or to a higher order) is spatially filtered. It is possible to demonstrate that, after the spatial filtering, each pulse of the input beam is converted into a composite output pulse, formed by a train of individual pulses. The number of individual pulses forming each composite pulse is equal to the number of lines of the grating which are illuminated by the input beam; the total duration of the composite pulse is equal to 21/c, in which 1 is the length of the projection of the grating onto the propagation axis of the input beam and c is the speed of light.
A temporal modulation of the intensity of each composite pulse, or train of individual pulses, is obtained by having, in front of the grating, an opaque screen in which an opening has been cut, the shape of which corresponds to the desired temporal modulation. This screen makes it possible to adjust the length of each line of the grating (measured perpendicularly to the plane of dispersion), and consequently the energy of each individual pulse of the output beam.
This technique is very simple to implement, but not very flexible: for each desired temporal profile, it is necessary to produce a new mask by cutting an opaque screen.
The document U.S. Pat. No. 6,577,782 discloses a refinement of the direct space-time shaping technique in which the opaque mask arranged in front of the grating is replaced by a programmable amplitude modulator, illuminated by the input beam and imaged on the diffraction grating. This assembly makes it possible to dynamically modify the temporal profile of the composite pulses, but is much more complex than the one initially proposed by C. Froehly and collaborators. Furthermore, neither of the two known variants of the direct space-time shaping technique makes it possible to modulate the phase and/or the polarization of the individual pulses, in addition to their amplitude.
The invention aims to remedy the above-mentioned drawbacks of the prior art.